Complexation of Ammonia Boranes with Al 3+ - ACS Publications

Apr 2, 2019 - Ammonia borane coordinated to Al(BH4)3 is known to dehydrogenate endothermally, showing the potential for direct hydrogen reversibility...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Complexation of Ammonia Boranes with Al3+ Iurii Dovgaliuk,*,†,‡ Kasper T. Møller,†,§ Koen Robeyns,† Véronique Louppe,† Torben R. Jensen,§ and Yaroslav Filinchuk*,†

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/02/19. For personal use only.

†

Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, place L. Pasteur 1, B-1348 Louvain-la-Neuve, Belgium ‡ Swiss−Norwegian Beamlines at the European Synchrotron Radiation Facility, 71 avenue des Martyrs, F-38000 Grenoble, France § Interdisciplinary Nanoscience Center and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: Ammonia borane, NH3BH3 (AB), is very attractive for hydrogen storage; however, it dehydrogenates exothermally, producing a mixture of polymeric products with limited potential for direct rehydrogenation. Recently, it was shown that AB complexed with Al3+ in Al(BH4)3·AB endothermically dehydrogenates to a single product identified as Al(BH4)3· NHBH, with the potential for direct rehydrogenation of AB. Here we explore the reactivity of AB-derived RNH2BH3 (R = −CH3, −CH2−) with AlX3 salts (X = BH4−, Cl−), aiming to extend the series to different anions and to enlarge the stability window for Al(BH4)3·NRBH. Three novel complexes were identified: Al(BH4)3·CH3NH2BH3 having a molecular structure similar to that of Al(BH4)3·AB but different dehydrogenation properties, as well as [Al(CH3NH2BH3)2Cl2][AlCl4] and [Al(NH2CH2CH2NH2)(BH4)2][Al(BH4)4], rare examples of Al3+ making part of the cations and anions simultaneously. The latter compounds are of interest in the design of novel electrolytes for Al-based batteries. The coordination of two ABs to a single Al atom opens a route to materials with higher hydrogen content.

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INTRODUCTION

Al(BH4)3 ·NH3BH3 → Al(BH4)3 ·NHBH + 2H 2

Solid-state hydrogen storage is one of the main challenges for the worldwide use of this environmentally benign fuel. In recent years, metal borohydrides [M(BH4)n]1,2 and M−B−N− H systems3,4 of metal amidoboranes (MABs), amine metal borohydrides, and complexes with ammonia borane, NH3BH3 (AB), have been intensively explored as the most attractive materials for potential solid-state hydrogen storage.5 AB is a promising hydrogen storage candidate because of its high hydrogen content (∼19.6 wt %) and acceptable stability upon transportation and storage.6,7 Pristine AB undergoes a stepwise decomposition, with 6.5 wt % hydrogen released below 112 °C and 14.5 wt % near 200 °C. However, both steps are accompanied by the release of undesirable borazine and the formation of stable polyaminoborane, (NH2BH2)n. A complicated mixture of the dehydrogenation products can hardly be regenerated in one step, unless a mixture of hydrazine/ ammonia is used.8,9 However, recently investigated metal borohydride−AB complexes, M(BH4)n(AB)m (n = 1, m = 1 or 2 for M = Li+; n = m = 2 for M = Ca2+ and Mg2+), showed more facile hydrogen desorption with less ammonia evolution compared to pure AB and MABs.10−13 Moreover, our recent investigation of the Al(BH4)3·AB complex established highpurity hydrogen release at moderate temperature (70 °C), according to the reaction14 © XXXX American Chemical Society

(1)

Al(BH4)3·AB is also encouraging in terms of a possible direct rehydrogenation of AB,14,15 which is typically regenerated via multistep chemical cycles.16 The striking property of Al(BH4)3·AB is endothermic dehydrogenation in the first decomposition step [reaction (1); ΔHdec = 39 kJ/mol, including melting], compared to the exothermic one for AB (ΔHdec = −22 kJ/mol on the first decomposition step, including melting).6 Endothermic dehydrogenation enables this system to be potentially reversible upon direct rehydrogenation, and the absence of complicated mixtures after dehydrogenation may allow for a viable chemical recycling of AB. Substitution of AB on the nitrogen side is an interesting way to prevent the second decomposition step of Al(BH4)3· NH3BH3, while retaining the endothermic character of the reaction (1). More precisely, the aim is to extend the stability range of Al(BH4)3·NHBH, which decomposes just above 90 °C, releasing heavier gas molecules and thus losing the potential for reversibility. We have chosen the lightest substituted AB, namely, methylamine borane, CH3NH2BH3 (MeAB), and ethylenediamine bis(borane), (CH2NH2BH3)2 Received: October 1, 2018

A

DOI: 10.1021/acs.inorgchem.8b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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The use of substituted AB in the form of a dimer, EDBB, leads to the slow formation of an en complex with the composition [Al(en)(BH4)2][Al(BH4)4]. This product suggests the elimination of diborane B2H6 from EDBB upon formation of the stable chelate complex with Al. Very low concentrations of en forming in situ upon complexation with Al(BH4)3 result in a complex with a very high Al(BH4)3-ton(en) ratio of 1:2. en-rich compounds Al(BH4)3·nen (1 ≤ n ≤ 4) were obtained by a direct reaction between the components,22,23 but the n = 1:2 ratio was not achievable by this method. The B−N bond splitting places the chemistry of this system far away from that of Al(BH4)3·AB. Most likely, these Al-EDBB-based systems are less stable and not suitable for reversible hydrogen storage. EDBB does not yield any products with AlCl3 at ambient conditions. Molecular and Crystal Structures of Al(BH4)3·MeAB. Al(BH4)3·MeAB crystallizes in the triclinic space group P1̅ [a = 6.2764(3) Å, b = 7.9566(5) Å, c = 10.3058(8) Å, α = 70.28(1)°, β = 74.74(1)°, and γ = 86.04(1)°]. It adopts a molecular structure and resembles the heteroleptic complex of Al(BH4)3·AB;14 see the comparison in Figure 1.

(EDBB). These AB derivatives can be easily obtained from commercially available chemicals and have previously been investigated as potential hydrogen carriers. However, the neat materials have several disadvantages for hydrogen storage. In particular, MeAB exhibits a high degree of volatility upon thermal treatment,17 and EDBB has a higher thermal stability compared to its parent counterparts, AB and MeAB.18,19 Moreover, in contrast to the numerous investigations on M(BH4)n(AB)m, there is no information about MeAB and EDBB analogues with borohydrides to date. The second improvement of the Al(BH4)3·AB model system can be achieved by substitution of the pyrophoric Al(BH4)3 by AlCl3. Exchange of the formally inert anion, however, should alter the electronic properties of the Al cation, as well as destabilize the structure containing a network of dihydrogen bonds.14 In this work, the expected AlX3·BH3NH2R (X = BH4−, Cl−; R = −H, −CH3, −CH2−) complexes were explored, and the products were characterized structurally. Three new compounds were obtained, with only one of them, Al(BH4)3·MeAB, adopting a molecular structure analogous to that of Al(BH4)3·AB. The other compounds, [Al(MeAB)2Cl2][AlCl4] and [Al(en)(BH4)2][Al(BH4)4], contain Al3+ in their cations and anions simultaneously; the ethylenediamine (en) complex forms via borane splitting from EDBB. Surprisingly, the reaction of AlCl3 with AB does not lead to the AB complex but yields NH4AlCl4.20 Overall, Al(BH4)3·MeAB is the most similar to Al(BH4)3·AB by composition and molecular structure. Thus, it has been studied in detail by in situ synchrotron powder X-ray diffraction (PXRD), thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry (TGA/DSC/MS), temperatureprogrammed photographic analysis (TPPA), and volumetric methods in order to compare its hydrogen storage properties with those of Al(BH4)3·AB. We show that the proposed modifications apply to the model system, despite only in part allowing for the rational design of the AB series templated on Al3+. Interestingly, two AB molecules can be coordinated by a single Al3+ ion, potentially bringing the gravimetric capacity of these systems closer to practical.

Figure 1. Isolated Al(BH4)3·AB and Al(BH4)3·MeAB complexes, where the Al3+ cation coordinates three BH4− anions and one AB or MeAB molecule. Color code: Al, red; B, olive; N, blue; C, teal; H, gray.

Al atoms coordinate three BH4− anions and one MeAB molecule, forming a mononuclear complex. The Al3+ cation is linked via BH2 edges and hence adopts a distorted tetrahedral coordination made up of borohydride groups, while the AlH8 polyhedron has the shape of a snub disphenoid, similar to Al in Al(BH4)3·AB or Mg in Mg(BH4)2.14,24,25 The Al···B distances with the BH4− ions are in the narrow range of 2.22−2.24 Å, similar to 2.21−2.23 Å in the two Al(BH4 ) 3·NH 3 BH3 polymorphs. This distance is nearly identical with the Al···B distances of 2.22−2.26 Å in M[Al(BH4)4] (M = Li+, Na+, K+, NH4+, Rb+, Cs+) and [Ph3MeP][Al(BH4)4], where the Al3+ cation is also coordinated to eight H atoms.31−33 The interatomic Al···B contact involving the MeAB’s BH3 group is slightly longer (2.34 Å) than the distances to the BH4− anions, fully consistent with the elongation in Al(BH4)3·AB (2.31 Å). However, they remain much shorter than the metal− boron distances in metal borohydride−AB complexes, namely, 2.63−2.92 Å in (LiBH4)2·AB, LiBH4·AB, and Ca(BH4)2· (AB)2.10,11 Al−H bond distances vary accordingly: for the BH4− groups, they range from 1.76(2) to 1.80(2) Å, similar to those in Al-based complex hydrides,31−33 and vary from 1.89(1) to 1.96(1) Å, where the AB’s −BH3 group is involved.14 The molecules of Al(BH4)3·MeAB are linked via simple and bifurcated N−Hδ+···Hδ−−B dihydrogen bonds with H···H

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RESULTS AND DISCUSSION Below we describe the formation and structures of the new compounds, followed by a detailed characterization of the stability and dehydrogenation for a new molecular analogue of the model system Al(BH4)3·MeAB. Formation of Al Complexes with ABs. The interaction of AlX3 (X = BH4− and Cl−) with light amine boranes BH3NH2R (R = −H, −CH3, −CH2−) results in three new ABbased complexes (see the Experimental Section and Table 1). Single-crystal X-ray diffraction and PXRD reveal the formation of a single-phase Al(BH4)3·MeAB having a molecular structure identical with that of the model compound Al(BH4)3·AB. Replacing the borohydride group by chloride, with the use of pristine AB, leads to the formation of crystalline NH4AlCl4 (Figure S1).20 NMR spectroscopy studies of the reaction product, described in the Supporting Information, reveal no NH3BH3, BH4−, or aminodiborane [BH3NH2BH3]− anions but the presence of [NH3BH2NH3]+.21 Double substitution both on the N side of AB and on the anion site reveals a complex containing two MeAB coordinated to a single Al3+. The singlecrystal structure gives a composition [Al(MeAB)2Cl2][AlCl4], where tetrachloroalanate serves as a counterion. B

DOI: 10.1021/acs.inorgchem.8b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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N distances of 1.48(5) and 1.57(5) Å are nearly the same as those in the noncomplexed MeAB.17,36 [Al(en)(BH4)2][Al(BH4)4] crystallizes in the monoclinic crystal system, space group P21/c [a = 8.4168(5) Å, b = 12.0021(7) Å, c = 16.2933(12) Å, and β = 101.89(1)°]. Among the previously reported complexes of Al(BH4)3 with en, several compositions of Al(BH4)3·n(en) (1 ≤ n ≤ 4) have been proposed;22 however, the only crystal structure characterized to date is that for [Al(en)3][BH4]3·(en).23 Both complex ions in [Al(en)(BH4)2][Al(BH4)4] adopt distorted tetrahedral coordination for Al atoms with respect to B and N atoms (Figure 4). The ∠N···Al···N of 86.5(2)° is

distances of 2.10−2.42 Å and H···H−N angles of 130−160° (Figure 2). These values are slightly below the sum of the van der Waals distances of 2.4 Å and are in agreement with the directionality criterion accepted for the dihydrogen bonds.26

Figure 2. Al(BH4)3·MeAB molecules linked via dihydrogen bonds, depicted by violet dashed lines.

Crystal Structures of [Al(MeAB)2Cl2][AlCl4] and [Al(en)(BH4)2][Al(BH4)4]. According to the CCDC27 and ICSD databases,28 [AlCl2L2]+ (L-ligand) complexes are fairly rare (two examples are known to date),29,30 and the formation of an autoionized borohydride complex (containing Al in both cationic and anionic forms) was found for the first time. [Al(MeAB)2Cl2][AlCl4] crystallizes in the orthorhombic crystal system, space group Pbca [a = 12.5826(5) Å, b = 12.6510(5) Å, and c = 20.4039(8) Å]. The [Al(MeAB)2Cl2]+ cation adopts a slightly distorted tetrahedral coordination with ∠B···Al···B of 104.1(2)° and ∠Cl···Al···Cl of 108.5(6)°, comparable to ∠Cl···Al···Cl 107.9(7)−112.1(7)° in the [AlCl4]− anion (Figure 3). The 96.7(1)−125.3(1)° range of

Figure 4. [Al(en)(BH4)2]+ cation and [Al(BH4)4]− anion connected via bifurcated dihydrogen bonds (violet dashed lines). Color code: Al, red; B, olive; N, blue; C, teal; H, gray.

similar to those of other known bidentate chelates of Al.35 The B···Al···B angle of 118.1(3)° in the cation is close to tetrahedral, while the angles around Al in the [Al(BH4)4]− anion range from 98.9(3) to 135.4(3)°, typical for a slightly flattened tetrahedron. This geometry is identical in aluminum borohydrides of alkali metals M[Al(BH4)4] (M = Li+, Na+, K+, NH4+, Rb+, Cs+) and in [Ph3MeP][Al(BH4)4].31−33 [Al(en)(BH4)2]+ contains a chelate cycle involving en and two BH4− groups coordinated via BH2 edges (Figure 4). The Al···B distances of 2.16(7) and 2.17(8) Å in the cation are slightly shorter than 2.23(8)−2.27(8) Å in the [Al(BH 4 ) 4 ] − anion.31−33 The Al···N distances of 1.95(4) and 1.96(4) Å are typical (1.9−2.0 Å) for other Al-containing cations.35 The Al···H distances of 1.72(4)−1.77(5) Å in the cation and 1.79(4)−1.89(4) Å in the anion are correlated with the Al···B distances. The structure is stabilized by simple and bifurcated dihydrogen N−Hδ+···Hδ−−B bonds with H···H distances of 2.02−2.22 Å and H···H−N angles of 140−160°. Analysis of Al(BH4)3·MeAB Thermal Decomposition. Variable-temperature synchrotron radiation PXRD on Al(BH4)3·MeAB shows that Al(BH4)3·MeAB is a single phase that melts around 50 °C (Figure 5). According to TGA/DSC (Figure 6), the forming liquid decomposes in three welldefined steps, and no other crystalline products were observed upon heating up to 80 °C (Figure 5). Al(BH4)3·MeAB was investigated using TGA/DSC/MS, volumetric analysis, and TPPA (Figures 6−9). The general conclusion is that its decomposition is complicated and does not follow the hydrogen release mechanism of the parent Al(BH4)3·AB. In particular, the TGA/DSC/MS data suggest three distinct steps, while two decomposition steps have been observed for Al(BH4)3·AB. The first decomposition step initiates already at T = 50 °C and corresponds to an endothermic peak at 57 °C, which is in

Figure 3. Representation of the [Al(MeAB)2Cl2]+ cation in [Al(MeAB)2Cl2][AlCl4]. Color code: Al, red; B, olive; Cl, green; N, blue; C, teal; H, gray.

the Cl···Al···B angles is closer to the B···Al···B angle range in homoleptic M[Al(BH4)4] (M = Li+, Na+, K+, NH4+, Rb+, Cs+) and [Ph3MeP][Al(BH4)4]31−33 than to the nearly ideal tetrahedral coordination in the heteroleptic complex anion [Al(BH4)2Cl2]−.34 Al···Cl distances in the [Al(MeAB)2Cl2]+ cation of 2.11(1) and 2.14(1) Å are in good agreement with 2.1−2.3 Å in other cationic complexes of Al.35 The MeAB ligand coordinates to Al via bridging H atoms of the BH3 groups with Al···B and Al−H distances of 2.24(5)−2.26(4) and 1.8(2)−1.9(2) Å, respectively, much like those in the MeAB complex with Al(BH4)3 (see above). The C−N and B− C

DOI: 10.1021/acs.inorgchem.8b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Volumetric measurements on Al(BH4)3·MeAB samples heated from room temperature to 50 °C (blue), 85 °C (green), and 200 °C (red) (ΔT/Δt = 1 °C/min), revealing releases of ∼1, ∼3, and ∼5 mol of gas per mole of the complex, respectively.

Figure 5. Temperature evolution of in situ synchrotron PXRD patterns for Al(BH4)3·MeAB.

Figure 9. TPPA of Al(BH4)3·MeAB heated from room temperature to 80 °C (ramp rate = 1 °C/min) showing fast degradation of the sample in a narrow temperature range.

reactions between Al(BH4)3 and MeAB or thermal decomposition of either MeAB or Al(BH4)3 or both. The mass loss of ∼30.5 wt % observed by TGA on the first step corresponds to ∼1 mol of the released gas products seen by the volumetric measurements at 50 °C (Figures 6 and 8). This suggests an equimolar release of volatile product(s), such as methylamine (CH3NH2; expected mass loss = 26.3 wt %), HCN (22.8 wt %), and/or other species, such as diborane B2H6 (23.4 wt %). It is practically impossible to distinguish in the laboratory MS data the m/z values for HCN and B2H6 because of the resolution limitations (e.g., at 26 and 27).38 However, the investigated MS range of m/z 1−50 must show a hydrogen release (m/z 2) and B2H6 (or HCN) evolution (in the range of m/z 23−27).39 Hydrogen may originate from the decomposition of diborane in the transfer line to MS. The strongest peaks characteristic for CH3NH2 (m/z 28, 30, and 31) have not been observed, suggesting that MeAB or its organic parts are not leaving the complex. We are not certain about the thermolysis products, but the observed data can be rationalized by reaction (2), giving ∼24 wt % volatile products.

Figure 6. DSC (upper) and TGA (lower) data for Al(BH4)3·MeAB decomposition measured from room temperature to 200 °C (ramp rate = 1 K/min).

Al(BH4)3 ·CH3NH 2BH3 Figure 7. MS data for Al(BH4)3·MeAB decomposition measured from room temperature to 200 °C (ramp rate 1 = °C/min).

→ [HAl(BH4)2 ]·CH3NH 2 + B2H6

(2)

The second decomposition step takes place between 82 and 115 °C with a mass loss of another 19 wt % and a weak exothermic peak at T ∼ 82 °C. It is initiating a sudden increase of B2H6 desorption along with signals at m/z 12 and 13, indicative for HCN. Reaction (2), followed by a release of

good agreement with the melting point of the crystalline sample observed by in situ PXRD and is close to the melting point of pure MeAB (58−59 °C).37 Thus, the evolved gases during thermal treatment could come from intermolecular D

DOI: 10.1021/acs.inorgchem.8b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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less reactive counterions, as we attempted with Cl−. Indeed, the idea of the double substitution on Al(BH4)3·AB seems to be the right strategy, and the yield and purity of the other complexes reported here should be improved in order to investigate their hydrogen storage properties. Compact energy storage systems based on Al are of great interest not just for hydrogen storage.42 With respect to its potential battery applications, Al has almost 4 times higher volumetric energy density than Li. The autoionized Al complexes [Al(MeAB)2Cl2][AlCl4] and [Al(en)(BH4)2][Al(BH4)4] fall into the family of potential novel electrolytes for Al3+-based batteries, currently using mainly [AlCl4]− molten salts and ionic liquids.46 The latter Al(BH4)3·1/2(en) complex is a close analogue of Mg(BH4)2·(en), one of the best Mg2+ion conductors.47

HCN, can reach 50.4 wt % compared to the observed 49.5 wt % loss and the release of 4 mol of gas. Despite the fact that our volumetric experiment reached only 3 mol of the released gas at 85 °C, this value was limited by kinetics (Figure 8). The overall reaction for the two first decomposition steps can be presented as 2Al(BH4)3 ·CH3NH 2BH3 → [HAl(BH4)2 ]2 + 2B2H6 + 2HCN + 4H 2

(3)

Finally, the third decomposition step occurs in the temperature range of 115−145 °C with yet another loss of 16 wt %. Unfortunately, instrumental artifacts make the interpretation of the DSC signal impossible in this range. MS registered a slight increase at m/z 41 and 42 at ∼120 °C. These values have previously been reported for the decomposition of Al(BH4)3 and/or its derivative [HAl(BH4)2]2.40,41 Our initial TGA/MS experiment at a high ramp rate has shown a sudden evolution of gases from Al(BH4)3·MeAB, so that the lid on the Al2O3 crucible was blown off at T ∼ 60 °C, while TGA also indicated a sudden event. To investigate this behavior further, TPPA was performed (Figure 9). The sample pellet begins to degrade at 50 °C in agreement with the observations in TGA/DSC/MS and the in situ PXRD. The pellet breaks in the narrow temperature range of 54−56 °C and nearly vanishes, leaving only a very small amount of powder in the bottom of the glass tube at 58 °C. This confirms that decomposition indeed occurs rapidly. We conclude that methyl substitution on the N side of AB leads to decomposition of Al(BH4)3, coming as the first step of the Al(BH4)3·MeAB thermolysis, instead of AB dehydrogenation in the model Al(BH4)3·AB. It is thus necessary to focus on other Al salts capable of coordinating to the modified AB. Thermal decomposition of these complexes may be better understood considering how strong the bonding interactions are. The bidentate coordination of the metal atom from two B−H bonds, M···H2B (η2), is known to be the most stable geometry in metal borohydrides, where the BH4 ligand bears a negative charge.5 In the case of neutral substituted AB molecules, the bond energy is expected to be lower but still significant in our case, considering the high charge and polarizing power of the Al3+ cation. The Al3+···H2B bonding energy is likely similar to that in σ-borane complexes,42 estimated at 20 kcal/mol,43 i.e., significantly higher than 4−6 kcal/mol per N−H···H−B interaction44 but lower than the interaction energy of the coordinate covalent B−N bond.45

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EXPERIMENTAL SECTION

Synthesis of MeAB and EDBB. The powders of MeAB and EDBB were obtained according to the simplified procedures described in the literature.15,48 These syntheses were performed using commercially available methylamine hydrochloride and ethylenediamine dihydrochloride (both from Sigma-Aldrich, 98%) and NaBH4 (Alfa Aesar, 97%). A total of 37 mmol of NaBH4 and stoichiometric amounts of methylamine hydrochloride or ethylenediamine dihydrochloride according to reactions (4) and (5) have been dissolved in 125 mL of tetrahydrofuran and were stirred for ∼24 h at room temperature.

CH3NH3+Cl− + NaBH4 → CH3NH 2BH3 + NaCl + H 2

(4)

(− CH 2NH3+Cl−)2 + 2NaBH4 → (− CH 2NH 2BH3)2 + 2NaCl + 2H 2

(5)

The obtained mixtures were filtered in air, and the solvent was removed from the filtrates with a rotary evaporator. Hexane was added to the resulting viscous liquids to precipitate crystals of MeAB or EDBB. PXRD confirmed the purity of the products (Figures S2 and S3). Interaction of AlCl3 with NH3BH3. AlCl3 (Sigma-Aldrich, 95%) and NH3BH3 (Sigma-Aldrich, 97%) were loaded under Ar in a 1:1 molar ratio into a ball milling jar and ground for 1 h, yielding a sticky product, where the starting AlCl3 is the only crystalline phase detected by diffraction at room temperature. The same behavior was observed with hand grinding of the mixture in an agate mortar. Additionally, solvent-mediated synthesis was performed in Et2O. A total of 2.63 mmol of AlCl3 was dissolved in 6 mL of Et2O and 2.59 mmol of NH3BH3 in 20 mL of Et2O, and the two solutions were mixed and stirred for 60 h at room temperature. The solution was filtered, and the filtrate was placed on a Schlenk line to dry. The sticky product could not be crystallized with n-hexane; thus, high vacuum (10−5 mbar) was applied, resulting in a slurry. Its PXRD analysis reveals the formation of NH4AlCl4 and an unknown monoclinic phase with nearly the same unit cell volume (Figure S1). Heating of this mixture results in the substantial growth of NH4AlCl4 diffraction peaks at around 60 °C (Figures S4 and S5), suggesting that it can be another polymorph of NH4AlCl4. We are not certain about the chemical reaction between AlCl3 and AB, stating only the appearance of NH 4 AlCl 4 from X-ray diffraction and the presence of [NH3BH2NH3]+ from solution NMR spectroscopy. Interaction of AlCl3 and MeAB. Stoichiometric amounts of MeAB and AlCl3 were mixed in an Ar atmosphere. The mixture readily melts/reacts at room temperature, giving a slurry that can be crystallized at −35 °C. Single-crystal X-ray diffraction reveals the formation of [Al(MeAB)2Cl2][AlCl4], according to the reaction

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CONCLUSION The present study shows that Al(BH4)3·AB can be used as a model system in the endeavor of designing hydrogen storage materials based on ABs. Indeed, substitutions on the N side of AB and on the anion site of AlX3 lead to stable complexes: Al(BH4)3·MeAB and autoionized [Al(MeAB)2Cl2][AlCl4] and [Al(en)(BH4)2][Al(BH4)4]. Coordination of two ABs to a single Al in the [Al(MeAB)2Cl2]+ cation opens a route to materials with higher hydrogen content. Despite the structural and compositional similarities of Al(BH4)3·AB and Al(BH4)3·MeAB, their hydrogen storage properties are very different. In contrast to Al(BH4)3·AB releasing H2 from the coordinated AB, Al(BH4)3·MeAB reveals the decomposition of Al(BH4)3 with a loss of B2H6. This makes clear the need to exchange the borohydride anion for

2AlCl3 + 2CH3NH 2BH3 → [Al(CH3NH 2BH3)2 Cl 2][AlCl4] (6) E

DOI: 10.1021/acs.inorgchem.8b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

All single-crystal data were solved by direct methods and refined by full-matrix least squares on F2 using SHELXL2014.50 PXRD. The powders of the prepared MeAB and EDBB, as well as the products of their reactions with AlCl3 and Al(BH4)3, were ground in an agate mortar inside an Ar-filled glovebox, and the powders were introduced into 0.7 mm glass capillaries and sealed with vacuum grease. Diffraction data were recorded on a MAR345 image-plate detector (Mo Kα radiation; λ = 0.71073; XENOCS focusing mirror). The obtained 2D images were azimuthally integrated by the program Fit2D using LaB6 as the calibrant.51 Variable-temperature in situ synchrotron radiation PXRD on Al(BH4)3·MeAB and AlCl3·AB samples was performed at the Materials Science Beamline (PSI in Villigen, Switzerland) using a Mythen II detector (λ = 0.776190 Å). The capillaries were heated from 30 to 80 to 300 °C (ΔT/Δt = 5 °C/min), respectively. TGA/DSC/MS Analysis. TGA and DSC analyses were made using a PerkinElmer STA 6000 apparatus simultaneously with MS analysis of the residual gas using a Hiden Analytical HPR-20 QMS sampling system. Samples (∼5 mg) were loaded into Al crucibles and heated from room temperature to 200 °C (ΔT/Δt = 1 °C/min) in an argon flow of 40 mL/min. The released gas was analyzed in the ranges m/z 1−19, 21−39, and 41−50, omitting the characteristic m/z 20 and 40 for Ar. Volumetric Study. Volumetric analysis was performed using a Hiden Isochema IMI-SHP analyzer. Decomposition experiments of the Al(BH4)3·MeAB complex were made with ∼60 mg of sample, in 5 bar of back-pressure of H2/He, and heated from 30 to 200 °C (ΔT/ Δt = 1 °C/min). The gas release was calculated from the calibrated volumes of the system. TPPA. The sample of Al(BH4)3·MeAB (∼10 mg) was pressed into a pellet and sealed under an Ar atmosphere in a glass tube placed in a custom-made Al heating block.52 The sample was heated from room temperature to 80 °C (ΔT/Δt = 1 °C/min). Photographs of the sample were collected every 1 min. NMR Spectroscopy. All NMR spectra were recorded at room temperature (295 K) on a Bruker Avance 500 spectrometer operating at 500.13 MHz for 1H, 160.46 MHz for 11B, and 130.32 MHz for 27Al. Experiments were run under the TopSpin program (1.3 version; Bruker) using a BBO{1H,X} probehead equipped with a z-gradient coil. 1H chemical shifts were referenced to the residual undeuterated C6D6 signal (7.16 ppm). 11B and 27Al NMR signals were referenced (δ = 0.00 ppm) according to the B and Al frequencies in the BF3·Et2O and Al(NO3)3 reference compounds. Standard zg programs were employed for 1D NMR experiments. An inverse-gated decoupling method with a Waltz-16 decoupling scheme using a pulse of 80 μs was used for 11B{1H} and 27Al{1H} NMR analyses.

However, this is not the only crystalline phase observed by PXRD, and we were not able to characterize in detail the other crystalline product(s) (Figure S6). Solution NMR spectroscopy shows that 1Hdecoupled 27Al NMR spectra are identical with the coupled ones, meaning there is no H directly next to Al, implying that the [AlCl2(BH3NH2Me)2] cation is not stable in solution. The AlCl4 anion is present as a peak at 70 ppm. A TGA/DSC/MS study shows a significant weight loss starting from ∼120 °C, producing CH3Cl (Figures S7−S9). Decomposed products obtained at 300 °C were not soluble in dimethyl sulfoxide and benzene. Interaction of AlCl3 with (CH2NH2BH3)2. In contrast to the spontaneous reaction of AlCl3 with MeAB, the same attempt with (CH2NH2BH3)2 in a 1:1 or 2:1 molar ratio yielded no new products at room temperature, as well as upon heating to 70 and 85 °C, respectively, resulting in only melts. Synthesis of Al(BH4)3·MeAB. Caution! All of the manipulations with highly pyrophoric Al(BH4)3 must be performed in a glovebox with an inert dry atmosphere! The reaction conditions to obtain Al(BH4)3· MeAB are similar to those used for the preparation of Al(BH4)3· NH3BH3.14 The amounts of reagents can be safely scaled up to 100− 200 mg of MeAB and 2−3 mL of Al(BH4)3, and PXRD analysis confirms the synthesis of a single-phase sample of Al(BH4)3·MeAB (Figure 5). Interaction of Al(BH4)3 with (CH2NH2BH3)2 and Formation of [Al(NH2CH2CH2NH2)(BH4)2][Al(BH4)4]. In order to study the interaction of Al(BH4)3 with EDBB, we used the same reaction procedure as that for the Al(BH4)3·NH3BH3 and Al(BH4)3·MeAB syntheses. In the reaction product, two types of crystal shapes were detected: flakes (major phase) and needles (minor phase). Structure determination on the needlelike crystals revealed the formation of [Al(en)(BH4)2][Al(BH4)4], suggesting the following reaction: 2Al(BH4)3 + (− CH 2NH 2BH3)2 → [Al(en)(BH4)2 ][Al(BH4)4 ] + B2H6

(7)

PXRD analysis of the sample reveals that the major phase is the starting EDBB, and either the fraction of [Al(en)(BH4)2][Al(BH4)4] is negligible or this complex is decomposing upon PXRD measurement. Table 1 contains a summary on the products formed in the studied systems.

Table 1. Identified Products Forming in the Systems AlX3− BH3NH2R [X = BH4−, Cl−; R = −H, −CH3, −(CH2)2NH2BH3] component NH3BH3 MeAB (CH2NH2BH3)2

Al(BH4)3 Al(BH4)3·NH3BH314 Al(BH4)3·MeAB [Al(en)(BH4)2] [Al(BH4)4]

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AlCl3 NH4AlCl4, [NH3BH2NH3]+ (NMR) [Al(MeAB)2Cl2][AlCl4]

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02630. Figures S1−S9 and Tables S1−S6 (PDF) NMR study data and spectra (PDF)

Single-Crystal X-ray Diffraction. Laboratory diffraction data for all of the measured single crystals were collected on a MAR345 imageplate detector (Mo Kα radiation; λ = 0.71073 Å; XENOCS focusing mirror). The crystals were loaded into inert grease in an Ar-filled glovebox and then measured at 150 K under a nitrogen flow (Oxford Cryosystems). The recorded data were indexed and integrated with CrysAlisPRO, and the implemented absorption correction was applied.49 The structure of [Al(MeAB)2Cl2][AlCl4] was solved in the orthorhombic space group Pbca with a = 12.5826(5) Å, b = 12.6510(5) Å, and c = 20.4039(8) Å. Al(BH4)3·MeAB crystallizes in the triclinic space group P1̅ with a = 6.2764(3) Å, b = 7.9566(5) Å, c = 10.3058(8) Å, α = 70.28(1)°, β = 74.74(1)°, and γ = 86.04(1)°. [Al(NH2CH2CH2NH2)(BH4)2][Al(BH4)4] has a monoclinic structure, space group P21/c with a = 8.4168(5) Å, b = 12.0021(7) Å, c = 16.2933(12) Å, and β = 101.89(1)°.

Accession Codes

CCDC 1871662−1871664 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: yaroslav.fi[email protected]. F

DOI: 10.1021/acs.inorgchem.8b02630 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ORCID

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Iurii Dovgaliuk: 0000-0003-1997-4748 Kasper T. Møller: 0000-0002-1970-6703 Torben R. Jensen: 0000-0002-4278-3221 Yaroslav Filinchuk: 0000-0002-6146-3696 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by FNRS (Grants PDR T.0169.13, EQP U.N038.13, and CdR J.0164.17). K.T.M. and T.R.J. acknowledge support by the Danish National Research Foundation, Center for Materials Crystallography (DNRF93), the Innovation Fund Denmark (HyFillFast), the Danish Research Council for Nature and Universe (Danscatt), and the Carlsberg Foundation. I.D. and K.T.M. thank Dr. Damir Safin for help with synthesis of the precursors. We are grateful for the allocated beam time at Materials Science Beamline, SLS/PSI, and to Dr. Nicola Casati for help. We thank Timothy Steenhaut for help with TGA/DSC−MS measurements and Dr. Gabriella Barozzino for help with NMR measurements.

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